1. Field of the Invention
The present invention relates to a laser processing method for processing a micro convexo-concave structure on a surface of a glass substrate and an optical diffraction element obtained by the laser processing method, and to a method for manufacturing an optical element including a diffraction grating which is used as a polarized light beam splitter, a coupling grating or the like, a diffraction type optical element which is used for a hologram, or an optical element such as a photonic crystal which is used as a birefringent plate, a light beam scatter plate or the like.
2. Description of Related Art
A glass plate has superior characteristics with respect to flatness, processing accuracy, weather resistance, heat resistance, etc. Therefore, there are already known devices such as a diffraction grating for use in optical communication and so on, or a micro lens for installation in a display device, which are formed by performing micro processing on a surface of a glass substrate.
For treating such micro processed regions on the glass substrate, conventionally, there is generally known a wet etching (chemical etching) method by using an etchant including hydrofluoric acid, etc., or a dry etching (physical etching) method by using a reactive ion etching, etc.
However, there is a problem with the care and the treatment of the etchant in the wet etching, and also a problem in the dry etching in that the facility of a vacuum container is necessitated, thereby requiring a large-scale facility by itself. Additionally, it is not cost-effective because a pattern mask and the like must be formed by a further complicated photolithography technique.
Moreover, an element for dividing wavelength, such as a diffraction grating, etc., which is available commercially at a relatively cheap price, is industrially produced by a method of obtaining an original negative plate by cutting a metal plate of aluminum or the like with a diamond blade (so called “a ruling engine”) and transferring upon an epoxy resin on the basis thereof.
In the above-mentioned industrial production method for the diffraction grating, a large-scale facility for the ruling is also necessitated, and as well, the element for dividing wavelength must be transferred onto organic materials for the mass production thereof. The transfer onto the organic materials shows good formability, however, it has a disadvantageous limit with respect to resistance against humidity and excessive temperatures.
On the other hand, it is known that a laser beam has strong energy so that the temperature of an irradiated surface of any arbitrary material increases resulting in ablation or evaporation of the irradiated portion thereof. Therefore, conventionally, a laser beam has been utilized in various processings or machining methods. In particular, the method of using a laser has been adapted to micro processing or machining because the laser beam can be easily focused onto a very fine spot.
Then, in the prior art, various methods for achieving such micro processing are already known, in which a periodic optical intensity distribution of the laser beam is obtained by causing a plurality of laser beams to mutually interfere. The mutually interfering beams are then radiated onto the surface of the material to be processed, such as a metal plate or the like, as disclosed for example, in Japanese Laid-open Patent Nos. Sho 50-42499(1975) and Hei 4-253583(1992), and in Japanese Patent Publication Nos. Hei 7-4675(1995), Hei 7-47232(1995), Hei 7-51400(1995), Hei 7-102470(1995), Hei 8-9794(1996), and Hei 8-25045(1996).
In particular among them, in Japanese Patent Publication No. Hei 8-25045(1996), a wave guide (a thin layer or film) having an index of refraction higher than air and that of the material to be processed is provided on the material to be processed, such as the metal plate or the like, and the laser beam is radiated onto the wave guide, thereby forming micro convexo-concave patterns in the wave guide by interference between the light beams transmitted in the wave guide and the radiated light beam, and providing a rainbow color developing function on the surface of the material to be processed.
Further, in one publication (“An Applied Physics”, by Masataka Murahara et al., Volume number 52, No. 1 (1983), in particular on page 84 thereof) it is reported that the micro convexo-concave structure of the organic thin film was directly produced by ablation of an organic macromolecule thin film, such as PMMA (polymethyl methacrylate) coated onto the glass substrate, using the interference light beam from an excimer laser.
In any one of the prior arts mentioned above, the thin film is formed on the surface of the substrate, thereby achieving a micro processing or machining thereon by absorbing the laser beam energy into the thin film to cause the ablation thereof. However, none of them takes into any consideration at all the energy of the laser beam.
Namely, although it was conventionally already known that a laser beam having an intensity higher than a certain level of energy must be irradiated to cause the ablation and so on, not only is the micro convexo-concave structure formed on the thin film, but also the substrate itself is processed or affected by the laser, in a case where the thin film is formed on the surface of the substrate. In particular, if the energy of the laser beam which reaches the substrate through the thin film is greater than a certain energy (threshold) that is enough to cause ablation on the substrate.
If micro processing on the substrate, which is different in physical property from the thin film, is being carried out at the same time, it cannot be used as an optical element, such as the diffraction grating, etc., for which is required a certain level of accuracy thereof.
Further, it has a disadvantage with respect to the characteristics of weather resistance and heat resistance, in the case where the thin film comprises organic macromolecules.
On the other hand, it is already known that a diffraction grating in which periodic convexo-concave structures are formed on a dielectric multiple film layer in one direction, as shown in FIG. 10(a), has superior characteristics as a polarized light beam splitter (Rong-Chung et al., OPTICS LETTERS Vol. 21, No. 10, p761, 1996).
Also, a diffraction grating in which periodic convexo-concave structures are formed on a dielectric multiple film layer in two directions, as shown in FIG. 10(b), has been proposed as a photonic crystal of three dimension (E. Yablonovitch, Journal of the Optical Society of America B Vol. 10, No. 2, p283, 1993).
At the present time, a dielectric multiple film layer itself has been widely used in various technical fields as a mirror, etc., and also various techniques have been already established as the method for manufacturing thereof, including an electron beam evaporation method, a heating evaporation method and a sputtering method.
Also, since the technology for forming the periodic convexo-concave structures on a dielectric multiple film layer is similar to the so-called patterning technology for producing VLSI (very large scale integration), etc., a diffraction grating, in which periodic convexo-concave structures are formed on a dielectric multiple film layer, can be produced by adopting the patterning technology for producing VLSI on a dielectric multiple film layer.
More concretely, as the technology for such patterning, there is known a wet etching (chemical etching) method by using an etchant including hydrofluoric acid, etc., or a dry etching (physical etching) method by using a reactive ion etching, etc., which is applicable thereto.
It is possible to manufacture a diffraction grating and so on by adopting the above-mentioned film forming and etching method. However, as is mentioned above, there is a problem with the care and the treatment of the etchant in the wet etching, and also a problem in the dry etching in that the facility of a vacuum container or the like is necessitated, thereby requiring a large-scale facility by itself. Additionally, in the dry etching, it is not cost-effective because a pattern mask must be formed by a further complicated photolithography technique including the steps of resist film painting, drying, exposure, baking, development and so on.
Moreover, when processing the etching on the dielectric multiple film layer laminated with plural kinds of layers, it is difficult to obtain a clear cross-section shape due to the difference in etching rates among the respective layers thereof.